Automatic volume control



Jam 4, 1944. E. sTANKo AUTOMATIC VOLUME CONTROL Filed July 9, 1942 mbtbh.. @Qu H1 :inventor 0L Stanlio Cttorncg Patented Jan. 4, 1944 AUTOMATIC VOLUME CONTROL Edward Stanko, Haddon Heights, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application July 9, 1942, Serial No. 450,239

4 Claims.

This invention relates to an automatic volume control for controlling the output volume from a set of loudspeakers in accordance with`=the noise level in the vicinity of the speakers. The

output level of the apparatus is set to operate a certain degree above the surrounding noise level and as the noise level varies, the output volume varies similarly so that the output of the speakers is maintained sulciently audible above the other noises while if the noise level decreases, the output level of the speakers decreases so that the output will not be annoying and loud.

The invention is of use in theatres where the noise level changes in accordance with the size of the audience and in connection with announcement systems in manufacturing plants Where the noise level varies from time to time depending on the number and Variety of the machines in operation.

One object of the invention is to provide an improved volume control. Another object is to provide an automatic volume control for maintaining the sound output of a group of loudspeakers above the ambient noise level.

Another object of the invention is to provide a thermionic amplier with automatic volume level controls. l

Another object of the invention is to provide a thermionic amplifier in which the output level will be controlled in accordance with an extraneous sound source.

Other and incidental objects of the invention will be apparent to those skilled in the art from a reading of the following specification and an inspection of the accompanying drawing in which the single figure of drawing is a schematic block diagram of' an amplifier, loudspeaker and volume control arrangement in accordance with my invention.

In the drawing, the sound to be reproduced is fed into the double pole, triple throw switch I2 either from the microphone I0, a photoelectric or electromechanical sound reproducer II or from an appropriate transmission line indicated above the switch I2. Any one of these sources may be selected by the switch, as desired. 'I'he output from the switch I2 is fed in the usual voltage amplifier I3 through the variable gain amplifier Il to the usual power amplifier I5, which is provided with a gain control, and thence to the speakers I6. It will be understood that a larger or smaller number of speakers may be used and that a plurality of power amplifiers may be used depending on the power requirements of the speaker arrangement used. The variable gain amplifier I4 is preferably o1' the type in which the gain may be either increased or decreased on increase in applied voltage depending upon the polarity of the applied voltage which is determined by the reversing switch 21. A vportion of the input to the voltage amplifier I3 is fed to an amplier Il and thence to a rectier I8. The output from the rectifier I8 is fed through the switch 21 to the variable gain amplier I4, referred to above. A portion of the output from the rectifier I8 is fed through a stabilizing lter I9 to an appropriate Voltage control indicator tube 20 and another portion of the output is fed through an appropriate switch to a volume indication meter 2|.

Within the eld of sound output of the speakers IB and also within the eld of disturbance caused by noise sources which interfere with the sound reproduction from the speakers I6, there is located a microphone pickup 22. The output of the microphone 22 is passed through an appropriate phasing switch 23 to an amplier 24 which is provided with an appropriate volume control. The output from the amplifier 24 is fed to the rectifier I8 in such polarity relation that it opposes the output from the amplifier I'I and it is the difference of these two outputs which is applied to the variable gain amplifier I4 and to the volume indicators 20 and 2l.

The two inputs to the rectifier I8 are put in proper phase relation by the phasing switch 23 or its equivalent. Since most noises, both auditorium noises and factory machine noises are usually of low frequencies, a low pass filter 25 which may, for example, pass substantially all frequencies below 200 cycles, while greatly attenuating frequencies above that value, may be pro vided.

In order to insure that the microphone 22 and amplifier 24 will be predominantly controlled from the noise sources rather than from the auditorium speakers, a high pass lter 26 greatly attenuating the frequencies passed by the filter 25 may be inserted at an appropriate point in the audio frequency channel.

It will be apparent from the foregoing that in the normal use of this apparatus the noise level around the microphone 22 will determine the gain of the amplifier I4 and thereby determine the output of the speakers I6. The apparatus may, however, be used for other purposes. For example, if the amplifier 24 is turned oft or the phasing switch 23 is left open, then the only input .to the rectifier I8 will be from the amplifier Il.

If the switch 21 is then turned to the opposite polarity of input to the amplifier Il from that used for controlling the speakers in accordance with the noise level, the circuit will act as a. compressor and the output from the speakers I6 will be rendered at a more uniform level than the input through the switch I2, the high amplitudes being lowered and the low amplitudes 1 being raised.

The apparatus may also be used to measure the frequency characteristics of an auditorium. If the switch 21 is left open so that the output of the rectifier does not affect the output of the speakers and if a variable frequency constant output record is reproduced by the apparatus I I. then the microphone 22 will pick up the effective sound level in the auditorium and the indicator ZI will then indicate the' difference between the input to the speakers I6 and the auditorium sound level produced thereby. If the characteristics of the various amplifiers, speakers and the microphone are known, then the readings of the meter 2| may be used directly to determine the auditorium frequency response.

In the first applications of this apparatus, the amount by which the sound level produced by the speakers I6 exceeds the noise level is determined by the gain control on the amplifier i5 and with the setting of this gain control, the variation in the output is determined by the variable gain amplifier I4.

I claim as my invention:

1. In combination a Variable gain amplifier, loudspeakers operated by the output from said amplifier, noise pick-up means, and means operated by said noise pick-up means and varying the gain of said amplifier in accordance with the noise level.

2. In combination a loudspeaker, an amplifier supplying audio frequency current to -said loudspeaker, sound pick-up means in the acoustic field of said speaker for picking up noise, and means controlled by the output of said sound pick-up means and controlling the output of said amplifier.

3. In combination a source of audio frequency impulses, a variable gain amplifier fed from said source, a power amplifier actuated by the output from the variable gain amplifier and loudspeakers operated by the output from the power amplifier, a second amplifier operated by the audio frequency impulses and feeding a rectifier connected to the variable gain amplifier, and a miorophone within the acoustic field of the loudspeakers and feeding the rectifier through an amplifier connected in opposition to the second amplifier whereby the gain of the variable gain amplifier is determined by the difference in the input to said rectier.

4. In combination a source of audio frequency impulses, a variable gain amplifier fed from said source, a power amplifier actuated by the output from the variable gain amplifier and loudspeakers operated by the output from the power amplifier, a second amplifier operated by the audio frequency impulses and feeding a rectifier connected to the variable gain amplifier. a microphone within the acoustic field of the loudspeakers and feeding the rectifier through an amplifier connected in opposition to the second amplifier whereby the gain of the Variable gain amplifier is determined by the difference in the input to said rectifier, and a reversing switch between the rectifier and the variable gain amplier.

EDWARD STANKO. 

